Add ink drop masking to TrayBackgroundView

ui/views/animation/ink_drop.h

Index: ui/views/animation/ink_drop.h
diff --git a/ui/views/animation/ink_drop.h b/ui/views/animation/ink_drop.h
index 1de593fef9213b4f5990ac8bf52dce177d620771..5a7722bc6ab92ecfba705b60aff3a5c1c3da2fa9 100644
--- a/ui/views/animation/ink_drop.h
+++ b/ui/views/animation/ink_drop.h
@@ -24,6 +24,9 @@ class VIEWS_EXPORT InkDrop {
  public:
   virtual ~InkDrop() {}
 
+  // Called by ink drop hosts when their size is changed.
+  virtual void HostSizeChanged(const gfx::Size& new_size) = 0;
+
   // Gets the target state of the ink drop.
   virtual InkDropState GetTargetInkDropState() const = 0;